Organic electroluminescent device

ABSTRACT

Provided is an organic electroluminescent device. The organic electroluminescent device comprises a first electrode, a second electrode, and at least two light emitting units disposed between the first electrode and the second electrode, wherein the light emitting units each comprises at least one light emitting layer, and a connection layer of a specific structure is further disposed between adjacent two light emitting units. By using a connection layer of a specific structure, the organic light-emitting device reduces the device voltage, prolongs life time of the device, and improves the device performance.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to Chinese Patent Application No. CN201910987157.0 filed on Oct. 17, 2019, the disclosure of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to an organic electroluminescent device.More specifically, the present disclosure relates to an organicelectroluminescent device including multiple light emitting units and aspecific connection layer.

BACKGROUND

The organic light-emitting device (OLED) is composed of a cathode, ananode and organic light emitting materials stacked between the cathodeand the anode, which, by applying voltages at both the cathode and theanode of the OLED, converts electric energy into light, and has theadvantages of wide angle, high contrast and fast response time. In 1987,Tang and Van Slyke of Eastman Kodak Company reported an organiclight-emitting device, which includes an arylamine hole transportinglayer and a tris-8-hydroxyquinoline-aluminum layer as the electrontransporting layer and the light emitting layer (Applied PhysicsLetters, 1987, 51 (12): 913-915). Once a bias is applied to the device,green light was emitted from the device. This invention has laid thefoundation for the development of modern organic light-emitting diodes(OLEDs). Since the OLED is a self-luminescent solid-state thin filmdevice, it provides great potential for display and lightingapplications.

In terms of device structure, OLEDs can be classified into conventionalOLEDs having a monolayer structure and OLEDs having a tandem structure(also called the stack structure). The conventional OLED includes onlyone light emitting unit between the cathode and the anode while thetandem OLED has multiple light emitting units stacked. One lightemitting unit generally includes at least one light emitting layer, ahole transporting layer and an electron transporting layer. Besides theabove-mentioned layers, the light emitting unit may further include ahole injection layer, an electron injection layer, a hole blocking layerand an electron blocking layer. Note that although the conventionalmonolayer OLED has only one light emitting unit, this light emittingunit may include multiple light emitting layers, for example, the lightemitting unit may include a yellow light emitting layer and a blue lightemitting layer. However, each light emitting unit includes only one pairof hole transporting layer and electron transporting layer. The tandemOLED includes at least two light emitting units, that is, the tandemOLED includes at least two pairs of hole transporting layer and electrontransporting layer. Herein, multiple light emitting units are arrangedin a vertically-stacked physical form, which realizes a tandemcharacteristic on the circuit, thus this kind of OLEDs is referred to asthe tandem OLED (in terms of the circuit connection) or the stacked OLED(in terms of the physical form). That is, under the same brightness, thecurrent density required by the tandem OLED is smaller than that of theconventional monolayer OLED, thus the life time is prolonged. On thecontrary, at a constant current density, the brightness of the tandemOLED is higher than that of the conventional monolayer OLED, but itsvoltage increases accordingly. Adjacent light emitting units of thetandem OLED are connected by a charge generation layer, and the qualityof the charge generation layer directly affects parameters of the tandemOLED such as voltage, life time and efficiency. Therefore, the region ofthe charge generation layer is required to be able to effectivelygenerate holes and electrons and to smoothly inject the holes andelectrons to corresponding light emitting units, and is also required tohave greater transmittance in the visible light range, and at the sametime, it needs to have stable performance and easy to be prepared.

The charge generation layer generally includes two parts, namely, ann-type material and a p-type material, to generate electrons and holes,and the combinations reported are generally inorganic n-typemetals/inorganic metals, such as Ca/Ag and Al/Au (Appl. Phys. Lett.2005, 87, 093504). In the application No. US20160285025A1, the chargegeneration layer is formed by two metal layers, where the first metallayer incudes a metal selected from Li, Cs, Na, Ba, Ca, Mg and Al, andthe second metal layer incudes a metal selected from Au, Ag, Cu, Sn, Tiand Al. There are also reported combinations of metal-doped n-typeorganic materials/inorganic p-type metals or oxides, such as Mg:Alq₃/WO₃(Jpn. J. Appl. Phys. 43 6418) or Bphen:Li/HAT-CN (Appl. Phys. Lett.2008, 92, 223311). For this combination, the n-type material of thecharge generation layer commonly used consists of an alkali metal or analkali metal compound doped with an organic electron transportingmaterial with high electron mobility, such as CsN₃:Bphen, Li:Bphen,Mg:Alq₃, Li:Alq₃ and the like. The p-type material of the chargegeneration layer commonly used consists of organic hole transportingmaterials or metal oxides, such as MoO₃, WO₃, V₂O₅, HATCN and the like.The charge generation layer may also use n-type organic materials/p-typeorganic materials, such as F16CuPc/CuPc (J. Appl. Phys. 2007, 101,014509). In the above material expressions, the symbol “:” refers to amixture of two materials, and the symbol “/” refers to a laminate of twocomponents before and after this symbol.

The application No. US20080171226A1 discloses a white organic lightemitting device, which comprises a charge generation layer. In thecharge generation layer, the n-type material is composed of Cs₂CO₃ dopedin BCP, and the p-type material is composed of MoO₃. However, for theabove metal oxide, the vapor deposition temperature is too high, thetransmittance is poor, the conductivity is weak, and it is badlycompatible with organic films, so it is not ideal p-type material, andwould affect the overall efficiency of the device. In order to improvethe charge transport ability, the n-type material needs to be doped withan alkali metal such as Li. In this case, Li₃N is vapor-deposited, andin the process of vapor deposition, Li₃N is decomposed into Li and N₂(Organic Electronics, 42, 102). N₂ would affect the vacuum degree of thevapor deposition chamber, decrease the vacuum degree and thus affect thequality of the device. Besides, simultaneous deposition of organicelectron transporting materials and alkali metal materials would makethe vapor deposition process complex and cause the cross contaminationof materials in the vacuum chamber.

The U.S. Pat. No. 6,717,358B1 discloses an organic electroluminescentdevice comprising an electron-transporting layer which can improvevoltage stability and which is composed of, in sequence, an n-typematerial layer, an interfacial layer, and a p-type material layer.Although the electron-transporting layer can improve the voltagestability of the device, the n-type material is composed of an organicelectron transporting material doped with an alkali metal. In this case,on one hand, the vapor co-deposition of two materials makes the processcomplex, and on the other hand, the vapor deposition of the alkali metalwould affect the vacuum degree of the vapor deposition chamber. Thematerial of the interfacial layer is composed of oxides, sulfides,selenides, nitrides or carbides of certain metals, such as the compoundof metals titanium, zirconium, molybdenum and lead.

The application No. US20160141338A1 discloses a tandem device includinga charge generation layer which includes, in sequence, a holetransporting layer, an electron-hole generation layer, an active metallayer, and an electron transporting layer, where the electron-holegeneration layer consists of HATCN or a metal oxide. Although each layeris vapor-deposited separately and the process is simpler without vaporco-deposition, the charge generation layer has a large number of layersand complex structure, and the metal layer is not compatible with theorganic layer composed of HATCN with a lowest unoccupied molecularorbital (LUMO) energy level of only 4.8 eV, which means that the holeinjection ability is poor, thereby resulting in a fact that the voltageand life time cannot meet requirements. Therefore, this device needs tobe further improved.

In the literature Journal of Luminescence, 154, 345-349, the device ofthe following structure was prepared: ITO/PEDOT:PSS/PVK:PBD(70:30w/w):Ir(ppy)₃(1 wt %)/LiF(1 nm)/Al(10 nm)/HATCN/TAPC:PVK/PVK:PBD(70:30w/w):Ir(ppy)₃(1 wt %)/LiF/Al, among which the two layers,HATCN/TAPC:PVK, were used as the charge generation layer, all layersexcept LiF and Al were spin-coated, and the transmittance of the Al filmwith the thickness of 10 nm in the visible light range was poor,limiting the further improvement of device efficiency. The structure ofthe disclosed light emitting unit is too simple, so the deviceperformance needs to be further improved.

At present, although the tandem electroluminescent device has beenwidely studied, it still has some disadvantages, such as high voltage,no significant increase in life time, complicated preparation processand the like, which are mainly caused by performance defects of theconnection layer connecting the light emitting units.

SUMMARY

To solve the above problems, the present disclosure aims to provide atandem organic electroluminescent device to solve at least part of theabove-mentioned problems. The tandem organic electroluminescent devicedisclosed herein is provided with a specific connection layer betweentwo light emitting units, which reduces the device voltage, prolongslife time of the device, and improves the device performance.

According to an embodiment of the present disclosure, disclosed is anorganic electroluminescent device including:

a first electrode,

a second electrode, and

at least two light emitting units disposed between the first electrodeand the second electrode;

wherein the light emitting units each includes at least one lightemitting layer;

wherein a connection layer consisting of a metal layer and a bufferlayer is further disposed between at least one set of adjacent two lightemitting units, wherein the material of the buffer layer is an organicmaterial; and

wherein the light emitting unit in contact with the buffer layer furtherincludes a hole injection layer, and the hole injection layer of thelight emitting unit in contact with the buffer layer includes thematerial of the buffer layer.

According to another embodiment of the present disclosure, disclosed isan organic electroluminescent device including:

a first electrode,

a second electrode, and

at least two light emitting units disposed between the first electrodeand the second electrode;

wherein the light emitting units each includes at least one lightemitting layer;

wherein a connection layer consisting of a metal layer and a bufferlayer is further disposed between at least one set of adjacent two lightemitting units; and

wherein the material of the buffer layer is an organic material, and theorganic material has a lowest unoccupied molecular orbital (LUMO) energylevel of greater than 4.9 eV.

According to another embodiment of the present disclosure, a displayassembly comprising any organic electroluminescent device describedabove is further disclosed.

The tandem organic electroluminescent device disclosed in the presentdisclosure is provided with a specific connection layer between twolight emitting units, which reduces the device voltage, prolongs lifetime of the device, and improves the device performance. In the presentdisclosure, the connection layer is composed of a metal layer and anorganic buffer layer. The material used by the metal layer has a highelectron injection ability, is easy to be deposited and is compatiblewith the organic material of the buffer layer, and the connection layerdoes not require complex co-vaporization process, and the process issimple. Therefore, the tandem electroluminescent device with such theconnection layer can significantly improve the device performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a tandem organic electroluminescentdevice 100 according to a specific embodiment of the present disclosure.

FIG. 2 is a schematic diagram of a tandem organic electroluminescentdevice 200 according to another specific embodiment of the presentdisclosure.

FIG. 3 is a schematic diagram of a tandem organic electroluminescentdevice 300 according to another specific embodiment of the presentdisclosure.

FIG. 4 is a schematic diagram of a tandem organic electroluminescentdevice 400 according to another specific embodiment of the presentdisclosure.

FIG. 5 is a schematic diagram of the structure of a conventionalred-light device.

DETAILED DESCRIPTION

The OLED can be fabricated on various types of substrates such as glass,plastic and metal. FIG. 1 schematically shows a sectional diagram of atandem organic electroluminescent device 100 without limitation. Thediagram is not necessarily drawn in scale, and some of layers can beomitted in the diagram as needed. The tandem organic electroluminescentdevice 100 includes a substrate 110, a first electrode 120, a firstlight emitting unit 130 a, a second light emitting unit 130 b, a secondelectrode 160, and a connection layer 140, where the connection layer iscomposed of a metal layer 140 a and a buffer layer 140 b. The substrate110 may be a substrate having a high transmittance or a bendablesubstrate, such as glass, plastic or metal. OLED devices may also beclassified into bottom-emitting OLED devices and top-emitting OLEDdevices in terms of structure. The bottom-emitting OLED device refers tothe OLED device emitting light from the substrate, and the top-emittingOLED device refers to the OLED device emitting light from the secondelectrode of the device.

As used herein, “top” means being located furthest away from thesubstrate while “bottom” means being located closest to the substrate.In a case where a first layer is described as “being disposed over” asecond layer, the first layer is disposed further away from thesubstrate. There may be other layers between the first and secondlayers, unless it is specified that the first layer is “in contact with”the second layer. For example, a cathode may be described as “beingdisposed over” an anode, even though there are various organic layersbetween the cathode and an anode. Similarly, the expression that thefirst layer is “in contact with” the second layer means that there areno other layers between the first layer and the second layer.

The filming process of the metal film is as follows: on the substratesurface, atoms form uniform, small and moveable atom clusters, which arealso called “islands”; these islands continue to accept new depositedatoms and then merge with other islands to become bigger; in the processof merging, new islands are gradually formed on the substrate surfacewhich is vacated due to the merging, and then these new islands mergeagain, and the process repeats until a structurally continuous film isformed when all isolated islands are joined together, or a discontinuousfilm is formed when the isolated islands are not joined together. In anexemplary embodiment of the present disclosure, the buffer layer of theconnection layer is ytterbium, and the atomic radius of the metal,ytterbium, is 2.4 angstroms (Å) (https://www.lookchem.cn/yuansu/101/).When the thickness of the vapor-deposited film is 10 Å, which isequivalent to the thickness of two atoms, these two atoms can only formisolated islands, and in this case, the formed film is discontinuous.

The transmittance herein refers to the percentage of transmitted lightpenetrated through the detected film to the incident light, measured inthe range of visible light (380-780 nm). The transmittance of differentfilms usually varies with wavelength. The transmittance described hereinrefers to the maximum transmittance in the visible light range.

The light emitting unit herein refers to a unit of organic materiallayers that can emit light with a voltage or current applied, and thelight emitting unit may include one or more light emitting layers. Thelight emitting unit generally also includes one or more organic materiallayers to inject or transfer charge. For example, besides the lightemitting layer, the light emitting unit may further include at least ahole injection layer, a hole transporting layer, an electron blockinglayer, a hole blocking layer, an electron transporting layer and anelectron injection layer. For example, in an exemplary embodiment of thepresent disclosure, the light emitting unit close the anode is composedof, in sequence, a hole injection layer, a hole transporting layer, anelectron blocking layer, a light emitting layer, a hole blocking layerand an electron transporting layer, and the light emitting unit close tothe cathode is composed of a hole injection layer, a hole transportinglayer, an electron blocking layer, a light emitting layer, a holeblocking layer, an electron transporting layer and an electron injectionlayer. In an embodiment, one OLED device may be described as an OLEDdevice having an “organic layer” disposed between the cathode and theanode. The organic layer may include one or more layers.

The device fabricated in accordance with the embodiments of the presentdisclosure may be incorporated into various consumer products having oneor more electronic component modules (or units) of the device. Someexamples of such consumer products include flat panel displays,monitors, medical monitors, televisions, billboards, lights for indooror outdoor lighting and/or signaling, head-up displays, fully orpartially transparent displays, flexible displays, smart phones,tablets, phablets, wearable devices, smart watches, laptop computers,digital cameras, camcorders, viewfinders, micro-displays, 3-D displays,vehicles displays, and vehicle tail lights.

The work function of the metal herein refers to the minimum energyneeded to move an electron from the interior to the surface of anobject. The “work function of the metal” herein is represented byabsolute values (positive values), that is, the higher the numericalvalue, the more the energy needed to pull the electron to the vacuumlevel. As described herein, the magnitude of the “work function of themetal” means the magnitude of the absolute value. For example, “the workfunction of the metal is greater than 5 eV” means that the energy neededto pull the electron to the vacuum level is greater than 5 eV.

The numerical values of highest occupied molecular orbital (HOMO) andlowest occupied molecular orbital (LUMO) mentioned herein are calculatedby using the Gaussian 09 software, B3LYP method and 6-311 g (d) basisset. Levels of the “HOMO” and “LUMO” are represented by absolute values(positive values), that is, the greater the numerical value, the deeperthe level. For example, the LUMO level of HATCN is calculated to be 4.81eV by this method.

The “connection layer”, as used herein, is a layer disposed between twolight emitting units to provide electrons and holes. It consists of ametal layer and a buffer layer, in which the metal layer is in contactwith the electron transporting layer or electron injection layer of alight emitting unit, and the buffer layer is in contact with the holeinjection layer or hole transporting layer of an adjacent light emittingunit. The “connection layer” may be a part of a charge generation layer,or may be a charge generation layer. When the “connection layer” is apart of a charge generation layer, the connection layer can further formthe charge generation layer with the p-type material which generatesholes in the light emitting unit, where the metal material is used asthe n-type material to generate electrons, the buffer layer is used foroptimizing the interface, and the p-type material is the material of thehole injection layer of a next light emitting unit. When the “connectionlayer” is a charge generation layer, the metal material is used as then-type material to generate electrons, and the buffer layer is used asthe p-type material to generate holes, especially when the buffer layeris thick to form a continuous or near-continuous film.

The “buffer layer”, as used herein, is a layer having a function ofoptimizing the interface, optionally, having a function of generatingholes, and it is a part of the connection layer. When the “connectionlayer” is a part of a charge generation layer, the “buffer layer” has afunction of optimizing the interface, reducing interface defects, andensuring the smooth transport of carriers. When the “connection layer”is a charge generation layer, the material of the “buffer layer” as ap-type material has a function of generating holes as well as a functionof optimizing the interface. The material of the buffer layer herein isan organic material, preferably an organic material having a LUMO energylevel greater than 4.9 eV, more preferably a compound having a structurerepresented by Formula 1 or a quinone compound and derivatives thereof.

Definition of Terms of Substituents

Halogen or halide—as used herein includes fluorine, chlorine, bromine,and iodine.

Alkyl—contemplates both straight and branched chain alkyl groups.Examples of the alkyl group include methyl, ethyl, propyl, iso-propyl,n-butyl, sec-butyl, iso-butyl, t-butyl, n-pentyl, n-hexyl, n-heptyl,n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl,n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl,neopentyl, 1-methylpentyl, 2-methylpentyl, 1-pentylhexyl, 1-butylpentyl,1-heptyloctyl, and 3-methylpentyl. Additionally, the alkyl group may beoptionally substituted. The carbons in the alkyl chain can be replacedby other hetero atoms. Of the above, preferred are methyl, ethyl,propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, t-butyl, n-pentyl,and neopentyl.

Cycloalkyl—as used herein contemplates cyclic alkyl groups. Preferredcycloalkyl groups are those containing 4 to 10 ring carbon atoms, andinclude cyclobutyl, cyclopentyl, cyclohexyl, 4-methylcyclohexyl,4,4-dimethylcyclohexyl, 1-adamantyl, 2-adamantyl, 1-norbornyl,2-norbornyl and the like. Additionally, the cycloalkyl group may beoptionally substituted. The carbons in the ring can be replaced by otherhetero atoms.

Alkenyl—as used herein contemplates both straight and branched chainalkene groups. Preferred alkenyl groups are those containing 2 to 15carbon atoms. Examples of the alkenyl group include vinyl, allyl,1-butenyl, 2-butenyl, 3-butenyl, 1,3-butandienyl, 1-methylvinyl, styryl,2,2-diphenylvinyl, 1,2-diphenylvinyl, 1-methylallyl, 1,1-dimethylallyl,2-methylallyl, 1-phenylallyl, 2-phenylallyl, 3-phenylallyl,3,3-diphenylallyl, 1,2-dimethylallyl, 1-phenyl-1-butenyl, and3-phenyl-1-butenyl. Additionally, the alkenyl group may be optionallysubstituted.

Alkynyl—as used herein contemplates both straight and branched chainalkyne groups. Preferred alkynyl groups are those containing 2 to 15carbon atoms. Additionally, the alkynyl group may be optionallysubstituted.

Aryl or the aromatic group—as used herein includes non-condensed andcondensed systems. Preferred aryl groups are those containing 6 to 60carbon atoms, preferably 6 to 20 carbon atoms, more preferably 6 to 12carbon atoms. Examples of the aryl group include phenyl, biphenyl,terphenyl, triphenylene, tetraphenylene, naphthalene, anthracene,phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, andazulene, preferably phenyl, biphenyl, terphenyl, triphenylene, fluorene,and naphthalene. Additionally, the aryl group may be optionallysubstituted. Examples of the non-condensed aryl group include phenyl,biphenyl-2-yl, biphenyl-3-yl, biphenyl-4-yl, p-terphenyl-4-yl,p-terphenyl-3-yl, p-terphenyl-2-yl, m-terphenyl-4-yl, m-terphenyl-3-yl,m-terphenyl-2-yl, o-tolyl, m-tolyl, p-tolyl, p-(2-phenylpropyl)phenyl,4′-methylbiphenylyl, 4″-t-butyl p-terphenyl-4-yl, o-cumenyl, m-cumenyl,p-cumenyl, 2,3-xylyl, 3,4-xylyl, 2,5-xylyl, mesityl, andm-quarterphenyl.

The heterocyclic group or heterocycle—as used herein includes aromaticand non-aromatic cyclic groups. Hetero-aromatic also means heteroaryl.Preferred non-aromatic heterocyclic groups are those containing 3 to 7ring atoms which include at least one hetero atom such as nitrogen,oxygen, and sulfur. The heterocyclic group can also be an aromaticheterocyclic group having at least one heteroatom selected from anitrogen atom, an oxygen atom, a sulfur atom, and a selenium atom.

Heteroaryl—as used herein contemplates non-condensed and condensedhetero-aromatic groups that may include from 1 to 5 heteroatoms.Preferred heteroaryl groups are those containing 3 to 30 carbon atoms,preferably 3 to 20 carbon atoms, more preferably 3 to 12 carbon atoms.Suitable heteroaryl groups include dibenzothiophene, dibenzofuran,dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene,benzoselenophene, carbazole, indolocarbazole, pyridoindole,pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole,oxadiazole, oxadiazole, dioxazole, thiadiazole, pyridine, pyridazine,pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine,indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole,benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline,quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine,phenazine, phenothiazine, benzofuropyridine, furodipyridine,benzothienopyridine, thienodipyridine, benzoselenophenopyridine, andselenophenodipyridine, preferably dibenzothiophene, dibenzofuran,dibenzoselenophene, carbazole, indolocarbazole, imidazole, pyridine,triazine, benzimidazole, 1,2-azaborine, 1,3-azaborine, 1,4-azaborine,borazine, and aza-analogs thereof. Additionally, the heteroaryl groupmay be optionally substituted.

Alkoxy—is represented by —O-alkyl. Examples and preferred examples ofalkyl are the same as those described above. Examples of the alkoxygroup having 1 to 20 carbon atoms, preferably 1 to 6 carbon atomsinclude methoxy, ethoxy, propoxy, butoxy, pentyloxy, and hexyloxy. Thealkoxy group having 3 or more carbon atoms may be linear, cyclic orbranched.

Aryloxy—is represented by —O-aryl or —O-heteroaryl. Examples andpreferred examples of aryl and heteroaryl are the same as thosedescribed above. Examples of the aryloxy group having 6 to 40 carbonatoms include phenoxy and biphenyloxy.

Arylalkyl—as used herein refers to an alkyl group that has an arylsubstituent. Additionally, the arylalkyl group may be optionallysubstituted. Examples of the arylalkyl group include benzyl,1-phenylethyl, 2-phenylethyl, 1-phenylisopropyl, 2-phenylisopropyl,phenyl-t-butyl, alpha-naphthylmethyl, 1-alpha-naphthylethyl,2-alpha-naphthylethyl, 1-alpha-naphthylisopropyl,2-alpha-naphthylisopropyl, beta-naphthylmethyl, 1-beta-naphthylethyl,2-beta-naphthylethyl, 1-beta-naphthylisopropyl,2-beta-naphthylisopropyl, p-methylbenzyl, m-methylbenzyl,o-methylbenzyl, p-chlorobenzyl, m-chlorobenzyl, o-chlorobenzyl,p-bromobenzyl, m-bromobenzyl, o-bromobenzyl, p-iodobenzyl, m-iodobenzyl,o-iodobenzyl, p-hydroxybenzyl, m-hydroxybenzyl, o-hydroxybenzyl,p-aminobenzyl, m-aminobenzyl, o-aminobenzyl, p-nitrobenzyl,m-nitrobenzyl, o-nitrobenzyl, p-cyanobenzyl, m-cyanobenzyl,o-cyanobenzyl, 1-hydroxy-2-phenylisopropyl, and1-chloro-2-phenylisopropyl. Of the above, preferred are benzyl,p-cyanobenzyl, m-cyanobenzyl, o-cyanobenzyl, 1-phenylethyl,2-phenylethyl, 1-phenylisopropyl, and 2-phenylisopropyl.

The term “aza” in azadibenzofuran, aza-dibenzothiophene, etc. means thatone or more of the C—H groups in the respective aromatic fragment arereplaced by a nitrogen atom. For example, azatriphenylene encompassesdibenzo[f,h]quinoxaline, dibenzo[f,h]quinoline and other analogues withtwo or more nitrogens in the ring system. One of ordinary skill in theart can readily envision other nitrogen analogs of the aza-derivativesdescribed above, and all such analogs are intended to be encompassed bythe terms as set forth herein.

Alkyl, cycloalkyl, alkenyl, alkynyl, arylalkyl, heterocyclic groups,aryl, and heteroaryl may be unsubstituted or may be substituted by oneor more selected from the group consisting of deuterium, halogen, alkyl,cycloalkyl, arylalkyl, alkoxy, aryloxy, amino, a cyclic amino group,silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl,acyl, carbonyl, carboxylic acid, ether, ester, cyano, isocyano,thiolalkyl, sulfinyl, sulfonyl, phosphino and combinations thereof.

It is to be understood that when a molecular fragment is described asbeing a substituent or otherwise attached to another moiety, its namemay be written as if it were a fragment (e.g. phenyl, phenylene,naphthyl, dibenzofuryl) or as if it were the whole molecule (e.g.benzene, naphthalene, dibenzofuran). As used herein, these differentways of designating a substituent or attached fragment are considered tobe equivalent.

In the compounds mentioned in the present disclosure, hydrogen atoms maybe partially or fully replaced by deuterium. Other atoms such as carbonand nitrogen may also be replaced by other stable isotopes thereof. Thereplacement with other stable isotopes in the compounds may be preferreddue to their attribution to the enhancement of device efficiency andstability.

In the compounds mentioned in the present disclosure, multiplesubstitutions refer to a range that includes a double substitution, upto the maximum available substitutions.

In the compounds mentioned in the present disclosure, the expressionthat adjacent substituents can be optionally joined to form a ring isintended to mean that two radicals are joined to each other via achemical bond. This is exemplified by the following formula:

Furthermore, the expression that adjacent substituents can be optionallyjoined to form a ring is also intended to mean that, in the case whereone of the two radicals represents hydrogen, the second radical isbonded at the position at which the hydrogen atom is bonded, therebyforming a ring. This is exemplified by the following formula:

According to an embodiment of the present disclosure, disclosed is anorganic electroluminescent device including:

a first electrode,

a second electrode, and

at least two light emitting units disposed between the first electrodeand the second electrode;

wherein the light emitting units each includes at least one lightemitting layer;

wherein a connection layer consisting of a metal layer and a bufferlayer is further disposed between at least one set of adjacent two lightemitting units, wherein the material of the buffer layer is an organicmaterial; and

wherein the light emitting unit in contact with the buffer layer furtherincludes a hole injection layer, and the hole injection layer of thelight emitting unit in contact with the buffer layer includes thematerial of the buffer layer.

According to another embodiment of the present disclosure, disclosed isan organic electroluminescent device including:

a first electrode,

a second electrode, and

at least two light emitting units disposed between the first electrodeand the second electrode;

wherein the light emitting units each includes at least one lightemitting layer;

wherein a connection layer consisting of a metal layer and a bufferlayer is further disposed between at least one set of adjacent two lightemitting units; and

wherein the material of the buffer layer is an organic material, and theorganic material has a LUMO energy level of greater than 4.9 eV.

In the above embodiments, the light emitting layer of the light emittingunit may emit light of the same color or light of different colors.

In the above embodiments, the expression that “a connection layer isfurther disposed between at least one set of adjacent two light emittingunits” means that when there are only two light emitting units in theorganic electroluminescent device, a connection layer is disposedbetween the two light emitting units; and when there are three or morelight emitting units in the organic electroluminescent device, aconnection layer is disposed between a set of adjacent two lightemitting units or a connection layer is disposed between two or moresets of adjacent two light emitting units.

According to an embodiment of the present disclosure, the material ofthe buffer layer is a compound represented by Formula I, or a quinonecompound and a derivative thereof:

wherein the ring B represents a substituted or unsubstituted carbon ringhaving 3 to 30 ring atoms or a substituted or unsubstituted heterocyclicring having 3 to 30 ring atoms;

wherein n is selected from an integer from 0 to 4, and X₁, X₂ and X₃are, at each occurrence identically or differently, selected from thegroup consisting of:

wherein V and W are selected from CR₂R₃, NR₄, O, S or Se;

wherein Ar is, at each occurrence identically or differently, selectedfrom substituted or unsubstituted aryl having 6 to 30 carbon atoms, or asubstituted or unsubstituted heteroaryl having 3 to 30 carbon atoms;

wherein R, R₁, R₂, R₃, R_(a), R_(b), R_(c), R_(d), R_(e), R_(f), R_(g)and R_(h) are, at each occurrence identically or differently, selectedfrom the group consisting of: hydrogen, deuterium, halogen, a nitrosogroup, a nitro group, an acyl group, a carbonyl group, a carboxylic acidgroup, an ester group, a cyano group, an isocyano group, SCN, OCN, SF₅,a boryl group, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; and

wherein A is a group having an electron-withdrawing group, and for anyof the structures, when one or more of R_(a), R_(b), R_(c), R_(d),R_(e), R_(f), R_(g) and R_(h) occur, at least one of R_(a), R_(b),R_(c), R_(d), R_(e), R_(f), R_(g) and R_(h) is a group having anelectron-withdrawing group; wherein a preferred group having anelectron-withdrawing group is selected from the group consisting of: F,CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, an isocyano group, SCN, OCN,pentafluorophenyl, 4-cyanotetrafluorophenyl, tetrafluoropyridyl,pyrimidyl, triazine, and combinations thereof.

According to an embodiment of the present disclosure, R is selected fromthe group consisting of: F, CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, anisocyano group, SCN, OCN, pentafluorophenyl, 4-cyanotetrafluorophenyl,tetrafluoropyridyl, pyrimidyl, triazine, and combinations thereof.

According to an embodiment of the present disclosure, X₁, X₂ and X₃ are,at each occurrence identically or differently, selected from the groupconsisting of:

In this embodiment, * represents the position at which X₁, X₂ and X₃ arejoined to the ring B.

According to another embodiment of the present disclosure, the ring B isselected from the group consisting of the following structures:

wherein R′ represents mono-substitution or di-substitution, and is, ateach occurrence identically or differently, selected from the groupconsisting of: hydrogen, deuterium, halogen, a nitroso group, a nitrogroup, an acyl group, a carbonyl group, a carboxylic acid group, anester group, a cyano group, an isocyano group, SCN, OCN, SF₅, a borylgroup, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; and

adjacent substituents R′ can be optionally joined to form a ring.

In this embodiment, * represents the position at which X₁, X₂ and X₃ arejoined on the ring B.

According to an embodiment of the present disclosure, the material ofthe buffer layer is selected from the group consisting of:

According to an embodiment of the present disclosure, a metal in themetal layer is selected from the group consisting of: Yb, Li, Na, K, Rb,Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, La, Ce, Pr, Nd, Sm, Eu, Tb, Th, Dy, Ho,Er, Em, Gd, Lu, Y, Mn, Ag, and combinations of a plurality thereof.

According to an embodiment of the present disclosure, the metal layer isformed by vapor deposition of one elemental metal, or is formed by vaporco-deposition of two or more elemental metals.

In the above embodiment, the metal of the metal layer may be anelemental metal, namely a metal having a purity greater than 99%, suchas metal ytterbium having a purity greater than 99%. The metal layer mayalso be formed by vapor co-deposition of two or more metals, where thepurity of each metal is greater than 99%. The expression that the purityof each metal is greater than 99% means that when the metal layercomprises a single metal, the purity of this metal is greater than 99%;when the metal layer comprises multiple metals, the purity of each metalduring the vapor deposition is greater than 99%.

According to an embodiment of the present disclosure, the buffer layerhas a thickness ranging from 0.1 nm to 30 nm, preferably from 0.5 nm to20 nm, more preferably from 1 nm to 15 nm.

According to an embodiment of the present disclosure, the metal layerhas a thickness ranging from 0.1 nm to 20 nm, preferably from 0.5 nm to10 nm, more preferably from 1 nm to 5 nm.

According to an embodiment of the present disclosure, the metal layerhas a work function of less than 4 eV.

According to an embodiment of the present disclosure, the material ofthe buffer layer has a LUMO energy level of greater than 4.9 eV,preferably greater than 5 eV.

The LUMO energy levels of compounds 1 to 28 were calculated by theGaussian 09 software, B3LYP method and 6-311 g (d) basis set and shownin the following table.

LUMO Com- energy pound level No. (eV) Compound 5.49 1 Compound 4.92 2Compound 5.41 3 Compound 5.26 4 Compound 5.42 5 Compound 5.34 6 Compound5.08 7 Compound 5.21 8 Compound 5.33 9 Compound 5.84 10 Compound 5.80 11Compound 5.97 12 Compound 5.45 13 Compound 5.44 14 Compound 5.46 15Compound 5.44 16 Compound 5.40 17 Compound 5.24 18 Compound 6.17 19Compound 6.18 20 Compound 5.50 21 Compound 5.27 22 Compound 5.08 23Compound 5.28 24 Compound 5.62 25 Compound 5.39 26 Compound 4.94 27Compound 5.28 28 Compound 5.20 29

According to an embodiment of the present disclosure, transmittance ofthe connection layer in a visible light range is greater than 70%.

According to an embodiment of the present disclosure, the at least twolight emitting units may emit light of the same color, or may emit lightof different colors.

According to an embodiment of the present disclosure, at least one ofthe buffer layer and the metal layer forms a discontinuous film.

According to another embodiment of the present disclosure, a displayassembly comprising the organic electroluminescent device is furtherdisclosed.

A material of compound 1 was vapor-deposited in different thicknesses onthe surface of clean silicon wafers to serve as a buffer layer, and theaverage surface roughness (Ra) was measured by atomic force microscopyas shown in Table 1. The surface roughness of the original silicon waferis generally less than 0.5 nm, and the diameter of a small molecule isgenerally 0.1 nm. When a film with a thickness of 1 nm wasvapor-deposited, the roughness was measured to be 8.31 nm, much largerthan the roughness of the silicon wafer and the diameter of a smallmolecule, which indicates that the vapor-deposited film with thethickness of 1 nm was an island-shaped discontinuous film. When thethickness of the film increased to 10 nm, the surface roughness of thebuffer layer sharply decreased to below 2 nm, very close to the surfaceroughness of the silicon wafer itself, which indicates that this layerof film was intending to be continuous.

TABLE 1 Average surface roughness (Ra) of the buffer layer withdifferent thicknesses Thickness of the buffer layer [nm] 1 5 10 Ra [nm]8.31 3.01 1.92

Specifically, the surface roughness was 1 nm when the vapor-depositedthickness was 8.31 nm, 3.01 nm when the vapor-deposited thickness was 5nm, and 1.92 nm when the vapor-deposited thickness was 10 nm. It can beseen that when the thickness of the buffer layer was from 1 to 10 nm,the surface had a certain roughness. In conjunction with the devicestructure in FIG. 1 , in one aspect, such roughness enables the holeinjection layer of the second light emitting unit 130 b to be betterattached to the material of the buffer layer 140 b, and in anotheraspect, it can also complement the rough surface of the metal layer 140a to make the connection layer 140 to form a relatively dense film as awhole, which inhibits the generation of traps, reduces the recombinationat the interface, realizes better compatibility between the films, andallows electrons and holes generated in the connection layer to flow tothe light emitting units 130 a and 130 b respectively and smoothly.

The first electrode 120 is formed on the substrate 110. The firstelectrode may be a transparent anode of the organic electroluminescentdevice 100, such as indium tin oxide (ITO), indium zinc oxide (IZO),indium gallium zinc oxide (IGZO) and the like, and such electrode isgenerally used to prepare a bottom-emitting or transparent device. Thefirst electrode may also be a composite layer that implements specularreflection, such as a stack of ITO/silver (Ag)/ITO, and such electrodeis generally used to prepare a top-emitting device. The second electrode160 is disposed to face the first electrode 120, and is generally acathode of the organic electroluminescent device 100. The secondelectrode 160 may be one of, or a combination of two or more of,elements of aluminum, magnesium, silver, gold, calcium, or ytterbium,among which aluminum is generally used as the second electrode of thebottom-emitting device, while magnesium, silver or a magnesium silveralloy is generally used as the second electrode of the top-emittingdevice.

Two light emitting units 130 a and 130 b are formed between the firstelectrode 120 and the second electrode 160. The first light emittingunit 130 a and the second light emitting unit 130 b may emit light ofthe same color, or may emit light of different colors, such as a mixtureof orange light and blue light, so that the device emits white light.Although not shown in the figure, each light emitting unit at leastfurther includes a hole injection layer, a hole transporting layer, anelectron transporting layer and an electron injection layer.Furthermore, some light emitting units may further include one or moreof a hole blocking layer and an electron blocking layer. It should benoted that when the hole injection layer of the second light emittingunit is adjacent to the buffer layer of the connection layer, this holeinjection layer may further be combined with the connection layer toform a charge generation layer. The hole injection layer of the secondlight emitting unit may be composed of a single hole injection materialor hole transporting material, or may be composed of an organic holetransporting material doped with a dopant, and the dopant may be abuffer layer material. The thickness of each layer can be adjustedaccording to the optimization result. Each light emitting unit mayfurther include one or two light emitting layers that emit differentcolors.

The number of light emitting units between the cathode and the anode mayalso be three or more. As shown in FIG. 2 , the tandem organic lightemitting device 200 includes three light emitting units 230 a, 230 b,230 c between the anode 220 and the cathode 260 on the substrate 210,and a connection layer 240 is disposed between the first light emittingunit 230 a and the second light emitting unit 230 b, where theconnection layer 240 includes a metal layer 240 a and a buffer layer 240b. A connection layer 250 is disposed between the second light emittingunit 230 b and the third light emitting unit 230 c, where the connectionlayer 250 includes a metal layer 250 a and a buffer layer 250 b. Thelight emitting units 230 a, 230 b and 230 c may emit the same color, ormay emit three different colors, for example, red, green and bluerespectively, so that the device emits white light. The connectionlayers 240 and 250 may use the same material and have the same filmthickness, or may use different materials and have different filmthicknesses.

The metal layer 140 a in the connection layer 140 is used to generateand transport electrons. Therefore, the metal material has a uniqueadvantage, and can be selected from Yb, Li, Na, K, Rb, Cs, Fr, Be, Mg,Ca, Sr, Ba, Ra, La, Ce, Pr, Nd, Sm, Eu, Tb, Th, Dy, Ho, Er, Em, Gd, Lu,Y, Mn, Ag or combinations of more than one of the above metals, andpreferably, the work function of the metal layer 140 a is less than 5eV, more preferably less than 4 eV, such as Yb, Mg, Ca, etc. Thethickness of the metal layer 140 a ranges from 0.1 nm to 20 nm,preferably from 0.1 nm to 5 nm. The thinner the metal layer, the higherthe transmittance, so that the luminescence efficiency is improved.Since the thickness of the metal layer 140 a is very thin, adiscontinuous thin film is formed, which allow the surface to have acertain roughness so that the buffer layer 140 b can be better attachedto the metal layer 140 a, while maintaining excellent electron transportperformance. The buffer layer 140 b is an organic material with theability to generate or transport holes. The buffer layer 140 b may bethe dopant material used in the hole injection layer, or may be otherhole injection or transporting layer materials. The LUMO energy level ofthe buffer layer 140 b is greater than 4.9 eV, preferably greater than 5eV. The thickness of the buffer layer 140 b ranges from 0.1 nm to 30 nm,preferably from 0.1 nm to 15 nm. Since the thickness of the buffer layer140 b is also very thin, a discontinuous film is formed, so that thesurface also has a certain roughness, which is complementary to themetal layer 140 a. The buffer layer 140 b contains a compoundrepresented by Formula 1.

The connection layer 140 is formed between the light emitting units 130a and 130 b. When the tandem organic electroluminescent device 100operates, the metal layer 140 a can provide electrons to the first lightemitting unit 130 a close to the anode 120, and these electrons formexcitons with the holes injected by the anode 120. At the same time, thebuffer layer 140 b provides or transports holes to the second lightemitting unit 130 b close to the cathode 160, and these holes formexcitons with the electrons injected by the cathode 160. Therefore, in acase where the cathode 160 and the anode 120 inject only one electronand one hole, the device can generate two excitons by itself, so thatthe efficiency of the device can theoretically be up to twice as high asthat of a conventional single-layer OLED.

The connection layer 140 may be prepared in any suitable manner, such asvacuum vapor deposition, vacuum thermal evaporation, sputtering,solution spin-coating, ink-jet printing, organic gasification printing,etc. The metal layer 140 a can be prepared in a vacuum chamber specificto prepare metals, and the buffer layer can be prepared in a vacuumchamber specific to prepare organic materials. The advantage is that itavoids cross contamination between metals and organic materials duringpreparation, further improving device performance. At the same time, thequantity of evaporation sources can be further reduced by using thedopant in the hole injection layer as the buffer layer. Furthermore, thelayer which is in contact with the buffer layer is the hole injectionlayer of the second light emitting unit, and since these two layers aregenerally uses the same material (the material of the buffer layer isused as the dopant of the hole injection layer), the preparation process(usually the vapor deposition process) is more continuous, and theinterface between the films can be transitioned more smoothly.

EXAMPLES

The present disclosure will be described below in detail in conjunctionwith the following examples. Apparently, the following examples are onlyfor illustrative purposes and are not intended to limit the scope of thepresent disclosure. Based on the following examples, those skilled inthe art can obtain other examples of the present disclosure byconducting improvements on these examples.

Example 1: Preparation of a Red Tandem Organic Electroluminescent Device300 Including the Connection Layer of the Present Disclosure, as Shownin FIG. 3

First, a glass substrate 310 which was previously coated with apatterned Indium Tin Oxide (ITO) anode 320 with a thickness of 120 nmwas cleaned with ultrapure water, and the ITO surface was treated withUV ozone and oxygen plasma. The substrate was dried in a nitrogen-filledglove box to remove moisture, then mounted on a bracket and placed in avapor deposition chamber. Organic layers specified below weresequentially deposited through thermal evaporation on the ITO anode at arate of 0.01 to 5 Å/s at a vacuum degree of about 1*10⁻⁷ torr. The firstlight emitting unit 330 a was first vapor-deposited, including thefollowing steps. The compound HI was used as a hole injection layer(HIL) 331 a with the thickness of 100 Å. The compound HT was used as ahole transporting layer (HTL) 332 a with the thickness of 350 Å. Thecompound H-1 was used as an electron blocking layer (EBL) 333 a with thethickness of 50 Å The red host compound H-2 was doped with the reddopant compound D-1 and then was co-deposited as a light emitting layer(EML) 334 a, in which the dopant concentration was 2% and the totalthickness was 400 Å. The compound H-3 was used as a hole blocking layer(HBL) 335 a with the thickness of 50 Å which was deposited on the lightemitting layer. On the HBL, the compound ET and the compound EIL wereco-deposited as an electron transporting layer (ETL) 336 a, in which thecompound EIL accounted for 60% of the total weight of ETL layer and thetotal thickness of ETL layer was 350 Å. After that, sequentially, themetal Yb with the thickness of 15 Å was vapor-deposited as a metal layer340 a of a connection layer 340, and the compound 1 with the thicknessof 10 Å was vapor-deposited as a buffer layer 340 b of the connectionlayer 340. Then, the second light emitting unit 330 b wasvapor-deposited, including the following steps. The compound 1 and thecompound HT were vapor co-deposited as a hole injection layer 331 b withthe thickness of 100 Å, wherein the compound 1 accounted for 3% of thetotal weight of a p-type material layer (i.e., the hole injection layer331 b). Note that the hole injection layer 331 b, together with themetal layer 340 a and the buffer layer 340 b of the connection layer340, may also be considered as a charge generation layer. Then, thecompound HT with the thickness of 700 Å was deposited as a holetransporting layer 332 b, and the compound H-1 was deposited as anelectron blocking layer (EBL) 333 b with the thickness of 50 Å. Then,the red host compound H-2 was doped with the red dopant compound D-1 andthen was co-deposited as a light emitting layer (EML) 334 b, in whichthe dopant concentration was 2% and the total thickness was 400 Å. Thecompound H-3 was used as a hole blocking layer (HBL) 335 b with thethickness of 50 Å which was deposited on the light emitting layer. Onthe HBL, the compound ET and the compound EIL were co-deposited as anelectron transporting layer (ETL) 336 b, and finally, the compound EILwith the thickness of 10 Å was vapor-deposited as an electron injectionlayer (EIL) 337 b, in which the compound EIL accounted for 60% of thetotal weight of ETL layer and the total thickness of ETL layer was 350Å, and aluminum with the thickness of 120 nm was vapor-deposited as acathode 360. Note that the above device structure is only forillustrative and is not limited to the description of the presentdisclosure. For example, the hole injection layer in the first lightemitting unit 330 a may use the same structure as in the second lightemitting unit 330 b, and vice versa. In another example, the secondlight emitting unit 330 b may use host compounds and light emittingmaterials in other colors as well as corresponding matched transportingmaterials and device structures. After the device was prepared, thedevice was transferred from the vapor deposition chamber back to theglove box and packaged with a glass cover. Structure examples of thecompound 1, compound HI, compound HT, compound H-1, compound H-2,compound H-3, compound D-1, compound ET and compound EIL are as follows:

Comparative Example 1: Preparation of a Red Tandem OrganicElectroluminescent Device Comprising a Connection Layer Including Only aMetal Layer

The preparation method was the same as that in Example 1, except thatthe connection layer did not have the buffer layer 340 b formed by thecompound 1.

Table 2 shows the test results of Example 1 and Comparative example 1.The chromaticity coordinates were measured at brightness of 1000 cd/m²,the brightness, external quantum efficiency and voltage were measured atthe current density of 15 mA/cm², and the life time of the device wasthe time required to decay to 97% of the initial brightness at 1000cd/m².

TABLE 2 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Example 1 0.685 0.314 41832 6506 49.09  9.21 Comparative 0.685 0.31424656 6318 48.07 10.65 example 1

The chromaticity coordinates of Example 1 and Comparative example 1 wereidentical, which indicates that the buffer layer 340 b did not affectthe color of the device. The external quantum efficiency of Example 1was 49.09%, 1.02% higher than that of Comparative example 1. At thecurrent density of 15 mA/cm², the brightness of Example 1 was 6506 cd/m²while the brightness of Comparative example 1 was only 6318 cd/m², andthe voltage of Example 1 was 9.21 V while the voltage of Comparativeexample 1 was as high as 10.65 V, 1.44 V higher than Example 1, whichindicates that the use of the buffer layer 340 b can effectively reducethe device voltage and improve the efficiency. The life time of Example1 was 41832 hours at 1000 cd/m² while the life time of Comparativeexample 1 was only 24656 hours. The life time of the organicelectroluminescent device of Example 1 was 1.7 times that of Comparativeexample 1. This is because the discontinuous buffer layer 340 beffectively complements the discontinuous metal layer 340 a, so that itinhibits the generation of traps, reduces the recombination at theinterface and improves the life time of the device. Meanwhile, since thehole injection layer of the second light emitting unit also uses thematerial of the buffer layer, the interface transition is smoother,which is also conducive to reducing defects and improving deviceperformance. These results indicate that the organic electroluminescentdevice of the present disclosure has lower voltage and longer life time.

A conventional monolayer device 500, that is, a device using the devicestructure completely identical with the single light emitting unit 330 aor 330 b, was also prepared. As shown in FIG. 5 , layers weresequentially deposited through thermal evaporation on the ITO anode 510at a rate of 0.01 to 5 Å/s at a vacuum degree of about 1*10⁻⁷ torr (thisfigure is an illustration and does not include a substrate layer). Thecompound HI was used as a hole injection layer (HIL) 520 with thethickness of 100 Å. The compound HT was used as a hole transportinglayer (HTL) 530 with the thickness of 350 Å. The compound H-1 was usedas an electron blocking layer (EBL) 540 with the thickness of 50 Å. Thered host compound H-2 was doped with the red dopant compound D-1 andthen was co-deposited as a light emitting layer (EML) 550, in which thetotal thickness was 400 Å. The compound H-3 was used as a hole blockinglayer (HBL) 560 with the thickness of 50 Å which was deposited on thelight emitting layer. On the HBL, the compound ET and the compound EILwere co-deposited as an electron transporting layer (ETL) 570, andfinally, the compound EIL with the thickness of 10 Å was vapor-depositedas an electron injection layer (EIL) 580, and aluminum with thethickness of 120 nm was vapor-deposited as a cathode 590. Theperformance of the conventional single layer device is shown in Table 3:at a current density of 15 mA/cm², the brightness was 3027 cd/m², thevoltage was 4.83 V, the external quantum efficiency was 23.5%, the lifetime for decaying to 97% of the initial brightness at 1000 cd/m² was11764 hours, and the chromaticity coordinates were x=0.683 and y=0.316.Therefore, the brightness and external quantum efficiency of Example 1were more than twice the brightness and external quantum efficiency ofthe conventional single layer device, the voltage of Example 1 was 0.42V lower than twice the voltage of the conventional single layer device,and the life time was also more than three times the life time of theconventional single layer device.

TABLE 3 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Single layer device 0.683 0.316 11764 3027 23.5 4.83

Example 2: Preparation of a Tandem Organic Electroluminescent DeviceComprising Three Light Emitting Units and Connection Layers of thePresent Disclosure

The preparation method of Example 2 was the same as that of Example 1,except that a second connection layer 350 and a third light emittingunit 330 c were sequentially added between the second light emittingunit 330 b and the cathode, where except that the thickness of the holetransporting layer in 330 c was 800 Å, the remaining organic layers in330 c were exactly identical to the organic layers in 330 b in terms oforder, thickness, doping and material, and the metal layer 350 a andbuffer layer 350 b included in the second connection layer 350 were alsoexactly the same as the connection layer 340. Furthermore, the secondlight emitting unit 330 b herein did not include 337 b. For the devicestructure, reference may be made to FIG. 2 .

Comparative Example 2: Preparation of a Tandem OrganicElectroluminescent Device Comprising Three Light Emitting Units andConnection Layers without Buffer Layers 340 b and 350 b

The preparation method was the same as that in Example 2, except thatthe connection layer only had the metal layer.

Table 4 shows the test results of Example 2 and Comparative example 2.The chromaticity coordinates were measured at brightness of 1000 cd/m²,the brightness, external quantum efficiency and voltage were measured atthe current density of 15 mA/cm², and the life time of the device wasthe time required to decay to 97% of the initial brightness at 1000cd/m².

TABLE 4 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Example 2 0.687 0.312 70552 8827 67.40 14.03 Comparative 0.687 0.31237707 8381 67.39 16.48 example 2

The chromaticity coordinates of Example 2 and Comparative example 2 wereidentical, and the external quantum efficiency of Example 2 was 67.40%while the external quantum efficiency of Comparative example 2 was67.39%, that is, their external quantum efficiencies were basically thesame. At the current density of 15 mA/cm², the brightness of Example 2was 8827 cd/m² while the brightness of Comparative example 2 was only8381 cd/m². The voltage of Example 2 was 14.03 V, while the voltage ofComparative example was as high as 16.48 V, 2.45 V higher than that ofExample 2. The above results also show that, compared with the use ofthe metal layer alone, the addition of the buffer layer makes theconnection layer flatter as a whole, charge transfer is more effective,and therefore the voltage is lower. At the same brightness, the lifetime of Example 2 was 70552 hours while the life time of Comparativeexample 2 was only 37707 hours. The life time of the organicelectroluminescent device of Example 2 was 1.9 times that of Comparativeexample 2. The reason for the increase of the life time is as mentionedabove. Meanwhile, compared with the conventional single layer device, atthe current density of 15 mA/cm², the voltage was 0.46 V lower than 3times that of the conventional single layer device, and the brightnessand external quantum efficiency were close to 3 times that of theconventional single layer device, and at the same brightness of 1000cd/m², the life time was nearly 6 times that of the conventional singlelayer device.

Example 3: Preparation of a Yellow Tandem Organic ElectroluminescentDevice 400 Comprising the Connection Layer of the Present Disclosure, asShown in FIG. 4

The treatment process of the substrate was the same as that of Example1, and layers were sequentially deposited through thermal evaporation onthe ITO anode 420 at a rate of 0.01 to 5 Å/s at a vacuum degree of about1*10⁻⁷ torr. The first light emitting unit 430 a was firstvapor-deposited, including the following steps. The compound HI was usedas a hole injection layer (HIL) 431 a with the thickness of 100 Å. Thecompound HT was used as a hole transporting layer (HTL) 432 a with thethickness of 400 Å. Then, the yellow host compound H-3 was doped withthe yellow dopant compound D-2 and then was co-deposited as a lightemitting layer (EML) 433 a, in which the dopant concentration was 20%and the total thickness was 400 Å. The compound H-3 was used as a holeblocking layer (HBL) 434 a with the thickness of 100 Å which wasvapor-deposited on the light emitting layer. On the HBL, the compound ETand the compound EIL were co-deposited as an electron transporting layer(ETL) 435 a, in which the compound EIL accounted for 60% of the totalweight of ETL layer and the total thickness of ETL layer was 450 Å.After that, sequentially, the metal Yb with the thickness of 15 Å wasvapor-deposited as a metal layer 440 a of a connection layer 440, andthe compound 1 with the thickness of 10 Å was vapor-deposited as abuffer layer 440 b of the connection layer 440. Then, the second lightemitting unit 430 b was vapor-deposited, including the following steps.The compound 1 and the compound HT were vapor co-deposited as a holeinjection layer 431 b, in which the compound 1 accounted for 3% of thetotal weight of a p-type material layer. The compound HT with thethickness of 800 Å was vapor-deposited as a hole transporting layer 432b. The yellow host compound H-3 was doped with the yellow dopantcompound D-2 and then was co-deposited as a light emitting layer (EML)433 b, in which the total thickness was 400 Å. The compound H-3 was usedas a hole blocking layer (HBL) 434 b with the thickness of 100 Å whichwas deposited on the light emitting layer. On the HBL, the compound ETand the compound EIL were co-deposited as an electron transporting layer(ETL) 435 b, and finally, the compound EIL with the thickness of 10 Åwas vapor-deposited as an electron injection layer (EIL) 436 b, andaluminum with the thickness of 120 nm was vapor-deposited as a cathode460. The package of the device was the same as that in Example 1. Notethat the light emitting unit in this embodiment did not use an electronblocking layer, but still retained an electron injection layer and ahole injection layer indispensable to a basic light emitting unit. Thestructure example of the compound D-2 is as follows:

Example 4: Preparation of a Yellow Tandem Organic ElectroluminescentDevice Comprising a Connection Layer Including Buffer Layer 440 b withthe Thickness of 50 Å

The preparation method was the same as that in Example 3, except thatthe thickness of the buffer layer 440 b in the connection layer was 50Å.

Example 5: Preparation of a Yellow Tandem Organic ElectroluminescentDevice Comprising a Connection Layer Including Buffer Layer 440 b withthe Thickness of 100 Å

The preparation method was the same as that in Example 3, except thatthe thickness of the buffer layer 440 b in the connection layer was 100Å.

Example 6: Preparation of a Yellow Tandem Organic ElectroluminescentDevice Comprising a Connection Layer Including Buffer Layer 440 b withthe Thickness of 150 Å

The preparation method was the same as that in Example 3, except thatthe thickness of the buffer layer 440 b in the connection layer was 150Å.

Comparative Example 3: Preparation of a Yellow Tandem OrganicElectroluminescent Device Comprising a Connection Layer Including Only aMetal Layer

The preparation method was the same as that in Example 3, except thatthe material of the buffer layer 440 b was not vapor-deposited.

Table 5 shows the test results of Examples 3, 4, 5 and 6 and Comparativeexample 3. The chromaticity coordinates were measured at brightness of1000 cd/m², the brightness, external quantum efficiency and voltage weremeasured at the current density of 15 mA/cm², and the life time of thedevice was the time required to decay to 97% of the initial brightnessat 1000 cd/m².

TABLE 5 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Example 3 0.451 0.541 80512 18387 36.56 7.70 Example 4 0.454 0.53864174 18426 36.91 7.63 Example 5 0.458 0.535 63907 18275 36.84 7.46Example 6 0.456 0.537 66933 18387 36.85 7.43 Comparative 0.447 0.54448991 18570 36.68 8.72 example 3

In Examples 3 to 6, the thickness of the buffer layer 440 b wassequentially increased. The chromaticity coordinates were basically thesame. The life time each was more than 60000 hours, and when thethickness was 10 Å, the life time of the device was the highest,reaching 80512 h. The external quantum efficiency of the buffer layerwith different thicknesses was almost the same, and the peak value was36.91% when the thickness was 50 Å. The device voltage decreased as thefilm thickness increased. Such device performance allows users tooptimize and adjust a device according to their own demands. Forexample, they can use a buffer layer 440 b with the thickness of 10 Å toobtain the longest life time, or use a buffer layer 440 b with thethickness of 150 Å to obtain the lowest voltage. In Comparative example3, the device did not include the buffer layer 440 b, the color wasblue-shifted, and the life time was reduced to 48991 hours, 31521 hourslower than that of Example 3. The voltage of Example 3 increased to 8.72V, 1.29 V higher than that of Example 6. These results indicate that thedevice comprising a connection layer with a buffer layer according tothe present disclosure has lower voltage and longer life time.

In order to verify the function of the buffer layer with the LUMO energylevel greater than 4.9 eV in the connection layer of the presentdisclosure, HATCN (with the LUMO energy level of 4.81 eV) was used as abuffer layer and compared with the compound 1 (with the LUMO energylevel of 5.05 eV) in the connection layer of the present disclosure.

Example 7: Preparation of a Tandem Organic Electroluminescent DeviceComprising a Connection Layer Including the Buffer Layer of the PresentDisclosure

The preparation method was the same as that in Example 1, except thatthe compound 1 with the thickness of 100 Å was vapor-deposited after theYb metal layer with the thickness of 10 Å was vapor-deposited and thatthe second light emitting unit did not include the hole injection layer.Note that in this device structure, the connection layer composed of themetal layer Yb and the buffer layer vapor-deposited by the compound 1 isa charge generation layer, where the metal layer Yb is responsible forgenerating electrons, and the buffer layer vapor-deposited by thecompound 1 is responsible for generating holes.

Comparative Example 4: Preparation of a Tandem OrganicElectroluminescent Device Comprising a Connection Layer without a BufferLayer of the Present Disclosure

The preparation method was the same as that in Example 1, except thatthe compound HI (HATCN) with the thickness of 100 Å was vapor-depositedafter the Yb metal layer with the thickness of 10 Å was vapor-depositedand that the second light emitting unit did not include the holeinjection layer.

Table 6 shows the test results of Example 7 and Comparative example 4.The chromaticity coordinates were measured at brightness of 1000 cd/m²,the brightness, external quantum efficiency and voltage were measured atthe current density of 15 mA/cm², and the life time of the device wasthe time required to decay to 97% of the initial brightness at 1000cd/m².

TABLE 6 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Example 7 0.684 0.315 39992 6527 48.57 9.22 Comparative 0.686 0.31434255 6240 47.93 9.48 example 4

With comparison between Example 7 and Comparative example 4, thechromaticity coordinates were almost identical, and at 1000 cd/m², thelife time of Example 7 was as high as 39992 hours while the life time ofComparative Example 4 was only 34255 hours. The life time was increasedby 16.7%. At 15 mA/cm², the brightness and external quantum efficiencyof Example 7 were slightly higher than those of Comparative example 4,but the voltage of Example 7 was only 9.22 V, 0.26 V lower than that ofComparative example. The results of Example 7 and Comparative Example 4show that the use of a buffer layer with a deeper LUMO level in theconnection layer can increase the life time of the device and reduce thevoltage of the tandem device.

Example 8: Preparation of a Red Tandem Organic Electroluminescent DeviceComprising Two Light Emitting Units and a Connection Layer of thePresent Disclosure

The preparation method was the same as that in Example 1, except thatthe thickness of the hole transporting layer of the first light emittingunit 330 a was 400 Å, that the material of the buffer layer was thecompound 29, and that the hole injection layer of the second lightemitting unit 330 b was formed by vapor co-deposition of the compound 29and the compound HT.

The structure example of the compound 29 is as follows:

Table 7 shows the test results of Examples 8 and the conventional singlelayer device 500. The chromaticity coordinates were measured atbrightness of 1000 cd/m², the brightness, external quantum efficiencyand voltage were measured at the current density of 15 mA/cm², and thelife time of the device was the time required to decay to 97% of theinitial brightness at 1000 cd/m².

TABLE 7 15 mA/cm² 1000 cd/m² External Chromaticity Life quantumcoordinates time Brightness efficiency Voltage (x, y) (hr) (cd/m²) (%)(V) Example 0.684 0.315 89000 6386 47.8 9.84 8 Single 0.683 0.316 117643027 23.5 4.83 layer device

For Example 8 and the conventional layer device 500, the chromaticitycoordinates were basically the same, which indicates that differentbuffer layer materials would not affect the color of the device. At thecurrent density of 15 mA/cm², the brightness of the single layer devicewas 3027 cd/m², the voltage was 4.83 V, the external quantum efficiencywas 23.5%, and the life time for reducing the initial brightness to 97%at 1000 cd/m² was 11764 hours. Therefore, the brightness and externalquantum efficiency of Example 8 were more than twice the brightness andexternal quantum efficiency of the conventional single layer device, thevoltage of Example 8 was 0.18 V higher than twice the voltage of theconventional single layer device (±0.2 V is within the normalexperimental error range), which indicates that the voltage of alaminated device prepared using the above-mentioned buffer material isabout twice the voltage of the conventional single layer device, andthat the addition of the buffer layer material does not increase thevoltage. At 1000 cd/m², the life time of Example 8 reached 7.6 times thelife time of the conventional single layer device.

In summary, in the tandem OLED, at least one layer of conductive film isrequired between different light emitting units for carrier transport,so in the above examples, a structure that retains a metal layer wasadopted. The above results show that the connection layer composed ofthe metal layer and the buffer layer made of the organic material with adeep LUMO energy level can solve the problems of high voltage, low lifetime, and complicated preparation process of the tandem device in theexisting art. The connection layer of the present disclosure dispenseswith the complicated vapor co-deposition, simplifying the preparationprocess, and improving the overall performance of the device, especiallysignificantly improving the life time of the device.

The above are only preferred examples of the present disclosure. Forthose ordinary skilled in the art, according to the idea of the presentdisclosure, there will be changes in specific implementations andapplication scopes, and the content of this specification should not beconstrued as limitations to the present disclosure.

What is claimed is:
 1. An organic electroluminescent device, comprising:a first electrode, a second electrode, and at least two light emittingunits disposed between the first electrode and the second electrode;wherein the light emitting units each comprises at least one lightemitting layer; wherein a connection layer consisting of a metal layerand a buffer layer is further disposed between at least one set ofadjacent two light emitting units, wherein the material of the bufferlayer is an organic material; and wherein the light emitting unit incontact with the buffer layer further comprises a hole injection layer,and the hole injection layer of the light emitting unit in contact withthe buffer layer comprises the material of the buffer layer.
 2. Theorganic electroluminescent device according to claim 1, wherein thematerial of the buffer layer is a compound represented by Formula I, ora quinone compound and a derivative thereof:

wherein ring B represents a substituted or unsubstituted carbon ringhaving 3 to 30 ring atoms or a substituted or unsubstituted heterocyclicring having 3 to 30 ring atoms; wherein n is selected from an integerfrom 0 to 4, and X₁, X₂ and X₃ are, at each occurrence identically ordifferently, selected from the group consisting of:

wherein V and W are, at each occurrence identically or differently,selected from CR₁R₂, NR₃, O, S or Se; wherein Ar is, at each occurrenceidentically or differently, selected from substituted or unsubstitutedaryl having 6 to 30 carbon atoms, or a substituted or unsubstitutedheteroaryl having 3 to 30 carbon atoms; wherein R, R₁, R₂, R₃, R_(a),R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) and R_(h) are, at eachoccurrence identically or differently, selected from the groupconsisting of: hydrogen, deuterium, halogen, a nitroso group, a nitrogroup, an acyl group, a carbonyl group, a carboxylic acid group, anester group, a cyano group, an isocyano group, SCN, OCN, SF₅, a borylgroup, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; wherein A is a group having anelectron-withdrawing group, and for any of the structures, when one ormore of R_(a), R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) and R_(h) occur,at least one of R_(a), R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) andR_(h) is a group having an electron-withdrawing group; whereinpreferably, the electron-withdrawing group is selected from the groupconsisting of: F, CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, an isocyanogroup, SCN, OCN, pentafluorophenyl, 4-cyanotetrafluorophenyl,tetrafluoropyridyl, pyrimidyl, triazine, and combinations thereof. 3.The organic electroluminescent device according to claim 2, wherein Ris, at each occurrence identically or differently, selected from thegroup consisting of: F, CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, anisocyano group, SCN, OCN, pentafluorophenyl, 4-cyanotetrafluorophenyl,tetrafluoropyridyl, pyrimidyl, triazine, and combinations thereof. 4.The organic electroluminescent device according to claim 2, wherein X₁,X₂ and X₃ are, at each occurrence identically or differently, selectedfrom the group consisting of the following structures:


5. The organic electroluminescent device according to claim 2, whereinthe ring B is selected from the group consisting of the followingstructures:

wherein R′ represents mono-substitution or di-substitution, and is, ateach occurrence identically or differently, selected from the groupconsisting of: hydrogen, deuterium, halogen, a nitroso group, a nitrogroup, an acyl group, a carbonyl group, a carboxylic acid group, anester group, a cyano group, an isocyano group, SCN, OCN, SF₅, a borylgroup, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; and adjacent substituents R′ can be optionallyjoined to form a ring.
 6. The organic electroluminescent deviceaccording to claim 1, wherein the material of the buffer layer isselected from the group consisting of:


7. The organic electroluminescent device according to claim 1, wherein ametal in the metal layer is selected from the group consisting of: Yb,Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, La, Ce, Pr, Nd, Sm, Eu,Tb, Th, Dy, Ho, Er, Em, Gd, Lu, Y, Mn, Ag, and combinations of aplurality thereof.
 8. The organic electroluminescent device according toclaim 1, wherein the metal layer is formed by vapor deposition of oneelemental metal, or is formed by vapor co-deposition of two or moreelemental metals.
 9. The organic electroluminescent device according toclaim 1, wherein the buffer layer has a thickness ranging from 0.1 nm to30 nm, preferably from 0.1 nm to 15 nm; optionally, the metal layer hasa thickness ranging from 0.1 nm to 20 nm, preferably from 0.1 nm to 5nm; optionally, the metal of the metal layer has a work function of lessthan 4 eV; optionally, the material of the buffer layer has a LUMOenergy level of greater than 4.9 eV, preferably greater than 5 eV;optionally, transmittance of the connection layer in a visible lightrange is greater than 70%; optionally, the at least two light emittingunits are capable of emitting light of the same or light of differentcolors; optionally, at least one of the buffer layer and the metal layerforms a discontinuous film.
 10. An organic electroluminescent device,comprising: a first electrode, a second electrode, and at least twolight emitting units disposed between the first electrode and the secondelectrode, wherein the light emitting units each comprises at least onelight emitting layer; wherein a connection layer consisting of a metallayer and a buffer layer is further disposed between at least one set ofadjacent two light emitting units; wherein a material of the bufferlayer is an organic material, and the organic material has a lowestunoccupied molecular orbital (LUMO) energy level of greater than 4.9 eV.11. The organic electroluminescent device according to claim 10, whereinthe material of the buffer layer is a compound represented by Formula I,or a quinone compound and a derivative thereof:

wherein ring B represents a substituted or unsubstituted carbon ringhaving 3 to 30 ring atoms or a substituted or unsubstituted heterocyclicring having 3 to 30 ring atoms; wherein n is selected from an integerfrom 0 to 4, and X₁, X₂ and X₃ are, at each occurrence identically ordifferently, selected from the group consisting of:

wherein V and W are, at each occurrence identically or differently,selected from CR₁R₂, NR₃, O, S or Se; wherein Ar is, at each occurrenceidentically or differently, selected from substituted or unsubstitutedaryl having 6 to 30 carbon atoms, or a substituted or unsubstitutedheteroaryl having 3 to 30 carbon atoms; wherein R, R₁, R₂, R₃, R_(a),R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) and R_(h) are, at eachoccurrence identically or differently, selected from the groupconsisting of: hydrogen, deuterium, halogen, a nitroso group, a nitrogroup, an acyl group, a carbonyl group, a carboxylic acid group, anester group, a cyano group, an isocyano group, SCN, OCN, SF₅, a borylgroup, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; wherein A is a group having anelectron-withdrawing group, and for any of the structures, when one ormore of R_(a), R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) and R_(h) occur,at least one of R_(a), R_(b), R_(c), R_(d), R_(e), R_(f), R_(g) andR_(h) is a group having an electron-withdrawing group; whereinpreferably, the electron-withdrawing group is selected from the groupconsisting of: F, CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, an isocyanogroup, SCN, OCN, pentafluorophenyl, 4-cyanotetrafluorophenyl,tetrafluoropyridyl, pyrimidyl, triazine, and combinations thereof. 12.The organic electroluminescent device according to claim 11, wherein Ris, at each occurrence identically or differently, selected from thegroup consisting of: F, CF₃, OCF₃, SF₅, SO₂CF₃, a cyano group, anisocyano group, SCN, OCN, pentafluorophenyl, 4-cyanotetrafluorophenyl,tetrafluoropyridyl, pyrimidyl, triazine, and combinations thereof. 13.The organic electroluminescent device according to claim 11, wherein X₁,X₂ and X₃ are, at each occurrence identically or differently, selectedfrom the group consisting of the following structures:


14. The organic electroluminescent device according to claim 11, whereinthe ring B is selected from the group consisting of the followingstructures:

wherein R′ represents mono-substitution or di-substitution, and is, ateach occurrence identically or differently, selected from the groupconsisting of: hydrogen, deuterium, halogen, a nitroso group, a nitrogroup, an acyl group, a carbonyl group, a carboxylic acid group, anester group, a cyano group, an isocyano group, SCN, OCN, SF₅, a borylgroup, a sulfinyl group, a sulfonyl group, a phosphinoxy group,substituted or unsubstituted alkyl having 1 to 20 carbon atoms,substituted or unsubstituted cycloalkyl having 3 to 20 ring carbonatoms, substituted or unsubstituted heteroalkyl having 1 to 20 carbonatoms, substituted or unsubstituted arylalkyl having 7 to 30 carbonatoms, substituted or unsubstituted alkoxy having 1 to 20 carbon atoms,substituted or unsubstituted aryloxy having 6 to 30 carbon atoms,substituted or unsubstituted alkenyl having 2 to 20 carbon atoms,substituted or unsubstituted alkynyl having 2 to 20 carbon atoms,unsubstituted or unsubstituted aryl having 6 to 30 carbon atoms,substituted or unsubstituted heteroaryl having 3 to 30 carbon atoms,substituted or unsubstituted alkylsilyl having 3 to 20 carbon atoms,substituted or unsubstituted arylsilyl having 6 to 20 carbon atoms, andcombinations thereof; and adjacent substituents R′ can be optionallyjoined to form a ring.
 15. The organic electroluminescent deviceaccording to claim 10, wherein the material of the buffer layer isselected from the group consisting of:


16. The organic electroluminescent device according to claim 10, whereina metal in the metal layer is selected from the group consisting of: Yb,Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, La, Ce, Pr, Nd, Sm, Eu,Tb, Th, Dy, Ho, Er, Em, Gd, Lu, Y, Mn, Ag, and combinations of aplurality thereof.
 17. The organic electroluminescent device accordingto claim 10, wherein the metal layer is formed by vapor deposition ofone elemental metal, or is formed by vapor co-deposition of two or moreelemental metals.
 18. The organic electroluminescent device according toclaim 10, wherein the buffer layer has a thickness ranging from 0.1 nmto 30 nm, preferably from 0.1 nm to 15 nm; optionally, the metal layerhas a thickness ranging from 0.1 nm to 20 nm, preferably from 0.1 nm to5 nm; optionally, the metal of the metal layer has a work function ofless than 4 eV; optionally, the material of the buffer layer has a LUMOenergy level of greater than 4.9 eV, preferably greater than 5 eV;optionally, transmittance of the connection layer in a visible lightrange is greater than 70%; optionally, the at least two light emittingunits are capable of emitting light of the same or light of differentcolors; optionally, at least one of the buffer layer and the metal layerforms a discontinuous film.
 19. A display assembly, comprising theorganic electroluminescent device according to claim
 1. 20. A displayassembly, comprising the organic electroluminescent device according toclaim 10.